by

Scott Munter, Executive Director SRIABE FIE Aust CPEng NER

Australia has a long history of damaging earthquakes

Despite this, many people believe that our cities and towns are immune from earthquakes

Many Engineers do not appreciate the differences between designing for wind and

earthquake actions

The structure must be designed for both wind and earthquake loads

Design and detailing of reinforcement are inseparable when it comes to seismic actions

The SRIA’s new Guide to Seismic Design and Detailing of Reinforced Concrete Buildings

in Australia addresses this important issue

Top 10 worst Australian onshore earthquakes in modern times (ranked by cost, magnitude and damage)

(source Australian Geographic July 10, 2012)

1. Newcastle NSW 28 Dec 1989 (Magnitude 5.6) Public Holiday

2. Beachport SA 10 May 1897 (Magnitude 6.5)

3. Meckering WA 14 Oct 1968 (Magnitude 6.9) Public Holiday

4. Ellalong NSW 6 Aug 1994 (Magnitude 5.4)

5. Adelaide SA 1 Mar 1954 (Magnitude 5.5)

6. Warooka SA 19 Sept 1902 (Magnitude 6.0)

7. Meeberrie WA 29 Apr 1941 (Magnitude 7.2)

8. Tennant Creek NT 22 Jan 1988 (Magnitude 6.3-6.7)

9. Kalgoorlie-Boulder WA 20 Apr 2010 (Magnitude 5.0)

10. Cadoux WA 2 June 1979 (Magnitude 6.1)

Magnitude 5.6

One of Australia's worst natural disasters

Killed 13 people, hospitalised 160

A small intraplate event with soft soils intensifying

shaking

Boxing Day Public Holiday so few people in CBD

Several events had occurred previously

Estimated $4 billion of damage to 35,000 homes, 147

schools & 3000 buildings

Damage over 9,000 square kms with movement up to

800km away

The Newcastle Worker Club

Subsequently demolished & rebuilt

(Photo Courtesy Newcastle Library)

Newcastle, NSW (1989)

Isoseismal map

Created by McCue & BMR/AGSO/GA

Beachport, SA (1897) – Magnitude 6.5

Damage to Beachport Post Office

(image courtesy of AEES)

Slumping near Robe due to liquefaction

(image courtesy of AEES)

Meckering, WA (1968) – Magnitude 6.9

37 km fault line scarp Height of step 1.5 m Most structures damages

or completely destroyed

Duration 40 seconds at 10.59 am on Public Holiday

20 people injured and 50 buildings damaged

Epicentre 9km SW of town and felt over 700km radius

(2nd strongest onshore in Australia)

Largest earthquake in region since 1918

Felt in Brisbane, Gold Coast and Toowoomba

Note: Christchurch earthquake on 22 February 2011 - Magnitude M6.3

Date TimeDepth(kms)

Lat. Long. Magnitude

30/7/2015 9.41 53 25.54S 154.00E 5.3

1/8/2015 13.38 10 25.38S 154.29E 5.7

1/8/2015 14.46 0 25.39S 154.23E 5.1

(Image courtesy Geoscience Australia)

Fraser Coast, QLD (2015) – Magnitude 5.1 to 5.7

Australia is not immune from earthquakes. It is estimated that:

On average 1 shallow M5 earthquake every 2 years (equivalent to Newcastle, NSW)

On average 1 shallow M6 earthquake every 10 years (equivalent to Christchurch, NZ)

A major earthquake will generate the most

severe structural ductility demand experienced

by a building

For a rare event, in a low seismicity region,

peak ground acceleration may be nearly

4 times greater

10 20 50 100 200 500 1000 2000 5000 100001.0

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0.6

0.4

0.2

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0.1 0.05 0.02 0.01 0.001 0.0001

Average Return Period (years)

Annual Probability of Exceedance, PP

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Australia

There are fundamental differences between designing for wind and earthquake loads

Designers often undertake a quick earthquake base shear check, compare it to the wind design

actions, find that wind “governs”, and stop. This practice ignores the detailing requirements

necessary to achieve structural behaviour consistent with the earthquake design base shear.

BCA requires designers to consider both wind & earthquake as separate design events

For wind, elastic methods of analysis, design and detailing based on static forces may be safely

used.

Given the rare and extreme nature of earthquakes, for economic reasons, designers are largely

concerned about preserving life and preventing structural collapse

For most structures, this will require the structural system to resist the imposed deformation

inelastically over a number of load cycles.

The difference is the poorly understood concept of structural ductility

Source: FEMA

Allows structure to withstand seismic induced drift

Source: Christchurch

Art Gallery Bookshop

CCTV

AS 3600 provides designers with the minimum design rules for Australia’s lower seismicity region

Structural system description

Intermediate moment-resisting frames (moderately ductile) designed in accordance with this Standard and Paragraph C4 of this Appendix

3 0.67

Combined systems of intermediate moment-resisting frames and ductile shear walls designed in accordance with this Standard and Paragraphs C4 and C5 of this Appendix

3 0.67

Ordinary moment-resisting frames designed in accordance with the main body of this Standard

2 0.77

Limited ductile shear-walls designed in accordance with the main body of this Standard

2 0.77

Other concrete structures not listed above 1.5 0.77

Special Moment-Resisting Frames (referred to in AS 1170.4) are not covered by Australian Standards.

In this case, overseas Standards must be used.

Table C3 of AS 3600

Sp

Relying on structural ductility when designing for life safety results in

significant damage from inelastic deformation - typically results in

demolition of the structure as it will be unrepairable

Should consider whether building requires protection of irreplaceable

contents, has a post disaster function, or whether it should be

repairable

Highest level of protection is by base isolation but seldom used in

Australia due to low seismicity

Typical method is to use a more robust, regular structure with defined

load paths

Cost can be as little as 1% to 3% of the total construction cost

Lowest level of protection is the minimum requirements in the body of

AS 3600

Christchurch CBD

Strive for simplicity and clarity in designing load paths

Maintain strong focus on detailing of reinforcement

Earthquakes exploit the weakest link

Static loads calculated may not resemble actual earthquake

Earthquake actions are dynamic

Stiffness of structure changes

Understand the behavior of

each member

Provide simple load paths

Transfer beam - stiffness of

structure changes with cracking

Structural robustness is not well defined

No specific requirements in AS 3600

BCA deem to satisfy ignores (BCA

separate performance section covers

but is optional)

Progressive and disproportionate

collapse must be avoided

Redundancy is an important issue

Failure of one part should not lead to the

collapse of the entire structure

Failure of loadbearing shear wall did not result in collapse

of entire building

Hotel Grand Chancelor, Christchurch

Figures 36 and 37 from Seismic Guide

Hotel Grand Chancellor, Christchurch, NZ

Increases resistance of structural system to progressive collapse

Remains of car park floor – Old Newcastle Workers Club, NSW(Photo courtesy Cultural Collections, The University of Newcastle, Australia)

Simple Reinforcement Detailing Improves Life Safety

Punching shear failure at column Resultant progressive collapse

Bottom bars not adequately

anchored in the confined

region of the column

Failure of a beam column joint

Copthorne Hotel, Christchurch 2011

All parts of a structure (framing & components) must be able to accommodate the required drift

AS 1170.4 requires a drift capacity of 1.5%

At about 1.5% drift (AS 1170.4 requirement), the cover concrete will typically be lost and

confined core will be adequate

Overall responsibility should be taken by the Principal Designer

Coordinate design work of subcontractors for parts and components

Failure of CTV Building, NZ attributed to senior engineer not supervising work of others

Failure of precast connectionBedford Row Carpark

Christchurch, NZ

Failure of precast connectionCrown Plaza HotelChristchurch, NZ

Pipe joint failureSan Fernando Earthquake, 1971

(FEMA 74, 1994)

Restraint of longitudinal bars lost due to fitment failure

Hotel Grand Chancellor, Christchurch, NZ 2011

(Photograph courtesy Peter McBean)

Note no fitments evident over

depth of beam

Column fitments provide restraint of longitudinal reinforcement and allow drift of columns

Loss of confined core due to fitment failure

Kobe earthquake, Japan 1995

Walls must be adequately reinforced to provide ductile behaviour and allow for drift

Actual damage and crack patterns from wall models

(Henry et al., University of Auckland, 2015)

Gallery Apartments, Christchurch NZ

Fractured reinforcement at base of wall – Grid F

Boundary chord in compression

Boundary chord in tension

Lateral force

Boundary collectorTransferring shear

Floor as diaphragm

(after ATC/SEAOC briefing paper)

Boundary chord in compression

Lateral force

Boundary collectorTransferring shear

Transfer lateral forces to shear resisting elements

Not covered by AS 3600

Refer to ACI 318M-14 for guidance

Ensure connection to shear resisting elements is adequate

CTV Building, Christchurch, NZ

Must allow for an interstorey drift of 1.5dst

(where dst =1.5% of storey height)

Approx. 75mm total

38 x 102 RSC

50 x 10 shim tack welded to seating channel

Polythene sheeting slip layers between stair and support

Precast beam withcast-in-situ topping

Stahlton

72mm seating30mm seismic gap

ThioflexPolyethylene tubePrecast stair

Collapsed stairs to the Hotel Grand Chancellor(Photograph courtesy of Dunning Thornton Consultants Ltd, NZ)

Australia is not immune from earthquakes

On average 1 shallow M5 earthquake every 2 years

On average 1 shallow M6 earthquake every 10 years

Australia may be categorised in an area of low to moderate seismicity, but the

consequences of an event in a city area would be very high eg the Christchurch scenario

Wind and earthquake loads are different and each needs to be considered

Designers need to adopt basic seismic design principles and to understand that seismic

actions and detailing are very different in order to develop compliant structures

Reinforcement detailing is just as important as the seismic design itself and trying to

understand how the building might fail under earthquake actions

Do not ignore non-structural elements as they are just as critical as the primary

structure - a lack of compliance may void your PI

The Guide has checklists included to provide a summary of issues that should be

considered by all parties

The Guide has many references which can generally be downloaded for free

eg Canterbury Earthquakes Royal Commission Reports Volumes 1-3 inclusive